Very briefly the X80 comes over the bridge. I know it comes from Lakeside and goes into Bluewater because my mother-in-law uses it. I don't believe it goes on anywhere else and i certainly don't know if it starts at an earlier point than Lakeside. Equally i don't know how frequently it runs but its certainly regular as i have known people who work at Bluewater use it. From Bluewater plenty of buses go into Dartford. My MIL is out at the mo otherwise i would give her a call.
Just found this link http://www.londonbusroutes.net/times/X80.htm
If you want any more info about reliability etc let me know and i can ask my MIL tomorrow.
There is also a ferry from Tilbury over to Gravesend, then buses go from Gravesend to Dartford. I don't know if that may be of any use.